Field of the Invention
The present invention relates to an imaging optical system used as an image projection optical system of an image projection apparatus and the like.
Description of the Related Art
Image projection apparatuses are required to have an imaging optical system as a wide-angle-of-view image projection optical system in order to be capable of projecting a larger image at a shorter projection distance. In general, as such a wide-angle-of-view imaging optical system, a so-called retrofocus optical system is used in which a lens having a negative refractive power is disposed on a magnification conjugate side and a lens having a positive refractive power is disposed on a reduction conjugate side.
However, in such a retrofocus imaging optical system, its lens disposed on the magnification conjugate side tends to have a larger diameter as its angle of view increases. This tendency appears especially in cases, such as image projection optical systems, where a long backfocus and a high telecentricity are required. U.S. Pat. Nos. 5,357,289 and 7,009,765 disclose the following imaging optical systems capable of solving this problem.
U.S. Pat. No. 5,357,289 discloses an imaging optical system which causes an entering light to form an intermediate image therein to enable easily ensuring a sufficient backfocus and which performs aberration corrections independently in front of and behind the intermediate image (intermediate imaging position) to provide a satisfactory image quality.
U.S. Pat. No. 7,009,765 discloses an imaging optical system which not only causes an entering light to form an intermediate image therein and performs aberration corrections in front of and behind the intermediate image (intermediate imaging position), but also bends an optical path of the light by a reflective surface to make the optical system compact.
However, although U.S. Pat. Nos. 5,357,289 and 7,009,765 disclose that the aberration corrections are performed in front of and behind the intermediate imaging position, neither of them disclose specific configurations of the optical systems. In addition, in order to provide a high definition image with such imaging optical systems each forming the intermediate image, it is necessary to take into consideration correction of a high-order off-axis aberration and correction of axial (longitudinal) chromatic aberrations which are combined in front of and behind the intermediate imaging position.